Luminescence enhancement by symmetry-breaking in the excited state in radical organic light-emitting diodes

Organic π-conjugated radicals have recently joined the ranks of high-efficiency light-emitting materials; however, their light-emission mechanism is still a matter of debate. Here, the authors highlight a recently proposed luminescent enhancement mechanism and record-breaking efficiency of a radical organic light-emitting diode.

species.There are two strategies to stabilize organic carbon radicals (Fig. 1c) 11,12 .One is the delocalization of the lone pair to the π-conjugated system to form OPRs. One representative example is the triphenylmethane (TPM) radical.The lone pair delocalizes to the three phenyl rings and becomes stabilized by πconjugation.The other is the introduction of steric hindrance near the radical to prevent the chemical reaction.One representative example is the tris(2,4,6-trichlorophenyl)methane (TTM) radical.The substituted chlorine atoms on the phenyl rings act as steric hindrance, and thus, it is more difficult for other molecules to be close to the radical than the unsubstituted TPM radical, bestowing further stabilization to the TTM radical.The TTM radical is stable at room temperature in contact with air and has light-emitting properties, albeit at a low PLQE of several percent.However, the TTM radical is weak against photoexcitation and immediately undergoes decomposition.The recent development of OPRs has overcome these difficulties.The recently developed OPRs are summarized in Fig. 1d.The first TTM radical-based OLED was developed by Peng et al.They utilized a TTM radical substituted by one carbazole (1Cz), developed by Gamero et al., as a light emitter 6,13 .The TTM-1Czbased OLED showed deep red electroluminescence with a peak wavelength of 700 nm.However, the achieved maximum EQE was limited to 2.4%.The breakthrough of efficiency improvements was brought by Ai et al. 7 in 2018.They incorporated 3substituted-9-phenyl-9H-carbazole (3PCz) and 3-substituted-9-(naphthalen-2-yl)-9H-carbazole (3NCz) into the core TTM radical.TTM-3PCz and TTM-3NCz doped in solid 4,4-bis(carbazol-9-yl)biphenyl (CBP) films (3.0 wt%) exhibited high PLQEs of 60.4% at 695 nm and 85.6% at 707 nm, respectively.TTM-3NCz showed a much higher resistance than TTM in cyclohexane solution under photoexcitation.The PL half-life of TTM was ~20 s, while the PL intensity of TTM-3NCz did not change after several thousand seconds.In addition, TTM-3NCz and TTM-3PCz exhibited high thermal resistance, enabling OLED fabrication using a thermal evaporation process.OLEDs were fabricated using these radicals as light emitters.Surprisingly, a maximum EQE of 17% for TTM-3PCz and 27% for TTM-3NCz were achieved.This high EQE of 27% is close to the theoretical limit of 30%.Thus, Ai et al. proved that the IQE can reach 100% when using radical emitters 7 .This success was achieved by improvements in radical stability and PLQE.The spread of π-conjugation in the substituted TTM is greater than that of TTM, and the lone pair can delocalize more, which is the reason for the stability improvement.In the luminous efficiency, in general, a large overlap between the singly occupied molecular orbital located at the core TTM and the highest occupied molecular orbital located at the carbazole substituent is necessary for a large oscillation strength.In addition, Abdurahman et al. proposed the PLQE enhancement mechanism by intensity borrowing from an intense high-lying electronic transition to a weak low-lying chargetransfer electronic transition 9 .However, the mechanism of luminous efficiency improvement is still a matter of debate.
Luminous efficiency enhancement by symmetry breaking in the excited state Very recently, Murto et al. reported the synthesis of mesityl groupsubstituted TTM radicals (M 1 TTM, M 2 TTM, and M 3 TTM), and M 3 TTM, having the highest symmetry, showed the highest PLQE 10 , which is not explained by the above two mechanisms (Fig. 2a).They explained the mechanism by symmetry breaking in the excited state, where the dihedral angle of one mesitylated phenyl moiety is twisted significantly more than the other two moieties, producing a large transition dipole moment in M 3 TTM and enhancing the PLQE to 28% from 8% of that of the unsubstituted TTM radical in a film doped with CBP host (Fig. 2b).This proposed mechanism is versatile and can broaden the scope of molecular design for PLQE enhancement, and it is worth exploring other substituents to further improve the PLQE.They modified TTM-3PCz with two mesityl substituents (M 2 TTM-3PCz), resulting in an increase in the PLQE to 93% in a film doped with a CBP host.The OLED using M 2 TTM-3PCz was fabricated and showed a record-breaking EQE of 28% at a wavelength of 689 nm.However, efficiency roll-off was observed in the low current density region, and the efficiency decreased at high luminance.In the doped film, time transient PL measurements were performed, and the formation of the intermolecular charge-transfer complex between the luminescent radicals and CBP host was observed, resulting in long-lived excited states on the order of microseconds.In closed-shell systems, TTA, a long-lived triplet-totriplet reaction, is thought to be responsible for the efficiency roll-off at high current densities 14 .Even in the open-shell system of this study, reactions between doublet and doublet excitons can occur and cause roll-off.The interaction between radicals and host molecules will need to be studied in detail to avoid generating long-lived intermolecular CT states.They also synthesized a main-chain copolymer of mesitylated TTM and 9,9-dioctyl-9H-fluorene (PFMTTM) based on the postpolymerization radicalization of a nonradicalized precursor, which is the first report of a luminescent polymer with radicals embedded in the main-chain.PFMTTM showed significantly redshifted emission beyond 800 nm, which is expected to be a new near-infrared light source (Fig. 2c).

Outlook
While the present postpolymerization radicalization strategy did not convert all the parent units to their radical forms, as evident from the luminescence from nonradicalized units (Fig. 2c), alternative methodologies for improving the radicalization efficiency or luminescence efficiency, such as polymerization of preradicalized monomers, will lead to the birth of further sophisticated luminescent polymers in combination with the expansion of available monomers and host molecules to replace CBP.Until a decade ago, it was common for radicals not to emit light and could not be applied in OLEDs; however, the recent developments of OPRs overturned this common sense.The future emergence of green-and blue-emitting OPRs with improved stability will open up possibilities for multicolor display applications.

Fig. 1
Fig. 1 Schematic illustrations of energy level diagrams and organic π-conjugated radicals for OLEDs.Differences in emission mechanisms of a conventional closed-shell and b open-shell systems.Chemical structures of c conventional triphenylmethane (TPM) and tris (2,4,6-trichlorophenyl) methane (TTM) radicals and d recently developed stable light-emitting radicals for OLEDs operating in the open-shell system.

Fig. 2
Fig. 2 Organic π-conjugated radicals improved in luminous efficiency by symmetry breaking and polymerization as tools for shifting luminescent properties.a Chemical structures of M 3 TTM, M 2 TTM-3PCz, and PFMTTM radicals.b Computationally optimized geometries of the M 3 TTM radical at vertical and adiabatic excited states.D 1 emission is enhanced by excited-state symmetry breaking.c Normalized PL spectra of M 3 TTM, M 2 TTM-3PCz, and PFMTTM.Copyright: Modified from ref. 10 used under CC BY.